A machine tool for most applications commonly requires a supply system for feeding a machining fluid stored in a reservoir to a machining zone constituted by a tool member and a workpiece. The machining fluid generally serves as a coolant and cutting medium as well as a flushing medium which carries away machined chips, particles and other machining products from the machining zone to keep the latter refreshed while allowing it to undergo continued machining or cutting actions under satisfactory conditions.
In conventional fluid pumping systems used with a machine tool, it has been a common practice to utilize a pump driven at constant RPM by an electric motor with a constant input power and, when the quantity of the machining fluid delivered to the machining zone is to be regulated or modified, a check valve, return valve, relief valve and/or other similar valve arrangements have been employed to control the output pressure of the pump. Since the pump is speed driven at constant, it has had to be driven at its maximum rating to be effective to produce a maximum fluid pressure required in a given course of machining operation or among a plurality of desired machining operations to be carried out by the machine tool.
One problem associated with conventional systems of this sort is that the machining fluid, while being circulated through a fluid conduit system, tends to rise in temperature due to its frictional contact with the internal walls of the conduits. Such temperature rise is disadvantageous in that it produces alteration in machining quality and expansion of machine components with which the fluid comes directly or indirectly into contact and thus leads to unsatisfactory and inaccurate machining results. In addition, with prior systems in which an excessive output of the pump is regulated by valve arrangements there is considerable heat loss in the pump driving power because of the additional fluid conduits.